This invention relates to radar systems, and, more particularly, to a radar system suited for the identification of targets closely associated with background clutter.
In a radar system, a transceiver (transmitter/receiver) transmits a high-frequency signal into space. When the transmitted signal strikes an object, a reflected signal is produced. Some portion of the reflected signal returns to the transceiver and is detected.
If there were a single return signal, the analysis of radar signals would be simple and straightforward. In practice, there are large numbers of return signals detected simultaneously. The returns are generated by a wide variety of features in addition to potential objects of interest, termed targets. If the radar transceiver is aimed on a low trajectory to track low-flying aircraft targets, there are also small-amplitude returns from natural and manmade objects on the ground such as hills and trees, houses, etc. If the radar transceiver is aimed on a higher trajectory, returns can arise from natural objects such as birds and from natural phenomena such as moisture droplets, hail, etc. Those returns present in addition to target returns are generally termed "clutter" herein.
Target returns can sometimes be clearly distinguished from clutter by their appearance during visual inspection of the radar display screen, as when the target return has a much higher amplitude than the clutter returns. In other instances this visual distinction cannot be made readily, or it is desirable to have the capability to distinguish a target from clutter in an automated fashion. In military applications, the radar may be in an unmanned missile which must identify targets without any human assistance. In non-military applications, fully automated aircraft landing systems and flight controller systems to serve as a backup for human control are under development. Such control systems could make use of automated target identification radar systems.
One approach now available for the automated identification of targets among clutter is a radar system utilizing CFAR, or constant false alarm rate detector analysis techniques. In this approach, the amplitude of a return signal is compared to a threshold value which is a multiple of a background amplitude value associated with the clutter. If the return signal is more than this threshold value, the return is judged to be a target rather than clutter. While operable, the conventional CFAR technique does not reliably distinguish targets from clutter in many cases of practical importance, such as the case of a target mixed with ground clutter, wherein the target and the clutter have comparable radar return amplitudes. In another approach, a target is separated from clutter using a range gate or Doppler filtering. If the target is close to or immersed in the clutter, the range gating technique is not useful. If the target is moving at an approximately constant distance from the transceiver, the Doppler-based technique is not successful in identifying the target.
There is a need for an improved radar system that reliably distinguishes targets from clutter. The present invention fulfills this need, and further provides related advantages.